This application claims priority from Korean Patent Application No. 2002-78147, filed on Dec. 10, 2002, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a media access control (MAC) master for executing a multi-point control protocol (MPCP) in an Optical Line Termination (OLT) of an Ethernet Passive Optical Network (Ethernet PON).
2. Description of the Related Art
An Ethernet passive optical network (Ethernet PON) has been proposed to apply Ethernet having been generally used in a Local Area Network (LAN) to a general subscriber network.
FIG. 1 is a view showing the configuration of an Ethernet PON.
In an Ethernet PON, an Optical Line Termination (OLT) 110 interfaces the network and is passively connected to Optical Network Units (ONUs) 120a-120n. Through an optical splitter 130, the optical network units (ONUs) 120a-120n are each connected to individual subscriber sides.
An Ethernet PON differs from an asynchronous transfer mode passive optical network (ATM PON), by transmitting data in the format of an Ethernet frame. The Ethernet frame includes an 8-byte preamble with a logical link identification (LLID) field, for identifying the ONU.
In an Ethernet frame transported from the OLT to an ONU (hereinafter, called “downstream”), the LLID indicates the ONU receiving the Ethernet frame (LLID may include broadcast). In an Ethernet frame transported from an ONU to the OLT (hereinafter, called “upstream”), the LLID indicates the ONU transmitting the Ethernet frame.
FIGS. 2a and 2b are views of downstream and upstream traffic flows, respectively, in the Ethernet PON.
The downstream Ethernet frame is transmitted through optical fibers, divided into a plurality of sections by a passive splitter, and broadcasted to all ONUs. Each ONU checks the LLID information of the transmitted Ethernet frame and determines whether to accept the data based on the LLID information. For upstream traffic flow, each ONU transmits Ethernet frames upstream within its own allocated time period according to a grant signal provided by the OLT, so that the Ethernet frames sequentially reach the OLT without overlapping. The OLT checks the LLID information written on the preamble of the frame and recognizes the ONU that transmitted the Ethernet frame.
The multi-point control protocol (MPCP) is used to control PON transmission and satisfy the Ethernet MAC frame transmission standard of IEEE 802.3ah. An Ethernet PON defines and utilizes MPCP messages to perform functions associated with a PON. By using MPCP messages, ONU can be connected to the OLT, a grant signal for upstream data transmission can be transferred to the ONU or a report about the queue states of an ONU can be transferred to the OLT.
The MPCP messages are categorized into five types: a gate message from the OLT to an ONU regarding time and duration of a data transmission; a report message representing the sending queue state of an ONU, wherein the state represents an amount of data in the queue waiting to be transmitted; a register_req message that is sent to the OLT after initialization of an ONU, indicating a request for registration; a register message sent from the OLT to an ONU in response to the register_req message, containing an LLID value assigned to the ONU; and a register_ack message sent from an ONU to the OLT to confirm the contents of the register message.
A gate message is also used for transmission of report and general data by registered ONUs with LLIDs allocated thereto. The gate message is also used for the unregistered ONUs to send register_req message. This gate message is marked with a specific flag. And after the register message is sent downstream during the registration procedure, the OLT provides a gate message to the corresponding ONU so that the ONU can respond with a register_ack message.
When the OLT allocates the transmission time of an upstream Ethernet frame to each ONU, the OLT designates each ONU's transmission start time on the basis of Round Trip Time (RTT) measured from the OLT to each ONU. This time is designated, so that the Ethernet frames, including preamble and idle patterns, sent by different ONUs do not overlap at the moment of reaching the OLT due to any transmission latency of optical fibers or any processing latency. In order to accomplish this, both the OLT and ONU have timers. The ONU synchronizes its timer with the OLT timer by copying the timer value of the OLT included in the MPCP messages. The OLT detects RTT to each ONU, using the timer value included in an MPCP message transferred upstream. The ONU interprets transmission start time of the Ethernet frame by synchronizing the ONU timer with the OLT timer. Considering RTT to the ONU, the OLT controls the transmission start time of each ONU.